1. Field of the Invention
The present invention relates to radio communication transmitters, receivers, systems and methods employing wireless digital communications using ultra wide band (UWB) signaling techniques.
2. Description of the Background
There are numerous radio communications techniques for digital data. Most recently, wireless digital communications have been applied to mobile telephone systems, pagers, remote data collection, and wireless networking of computers as well as other applications. One book on the subject is “Wireless Digital Communications, Modulation & Spread Spectrum Applications,” by Kamilo Feher and another is “Digital Communications Fundamentals and Applications” by Bernard Sklar, Prentice-Hall, Englewood Cliffs, N.J., ISBN 0-13-211939-0, the entire contents of both being incorporated herein by reference. Among other things, these books deal with conventional modulation of a carrier with, for example, phase or frequency shift keying (i.e. FSK, MSK, GMSK, BPSK, DBPSK, QPSK, O-QPSK, FQPSK, π/4-DEQPSK, and pulse position modulation (PPM)). The American and Japanese cellular standard, for example, uses π/4-DEQPSK.
These systems conventionally use either time division multiple access (TDMA) or code division multiple access (CDMA) to share an allocated bandwidth between multiple users. Spread spectrum variants of these systems They use either FHSS (frequency hop spread spectrum) or the CDMA codes (a direct sequence approach) to spread the spectrum. “Spread spectrum” provides a way of sharing bandwidth between multiple users and also providing a robust signal that is relatively immune to background noise.
The spread spectrum technique improves the robustness of the signal being sent through a predetermined amount of repetition in the signal, relative to the data that is contained in the signal. Often, this redundancy is described in terms of the number of “chips” per data bit. Conventional spread spectrum systems, codes and techniques are described in “Spread Spectrum Signal Design LPE and AJ Systems”, by David L. Nicholson, Computer Science Press, 1988, ISBN 0-88175-102-2, the entire contents of which being incorporated herein by reference.
In such spread spectrum signals, the infraction of information in the chip is embodied in a predetermined number of carrier cycles such that conventional frequency analysis (spectral analysis such as FFT techniques) may be used in analyzing or receiving the signals. Such analysis and reception techniques presume a persistence of a time-continuous signal in order to provide optimum detection.
In any of these conventional radio frequency communication schemes, the data is used to modulate a carrier wave, typically in the microwave frequencies, so that the transmissions may be generated with relatively compact equipment and propagate efficiency in line of sight (LOS) communication channels. However, when the transmitted energy is concentrated at such high frequencies, the energy is easily blocked by terrain or other intervening objects that are present between the transmitter and the receiver. To appreciate why the blocking of radio frequency energy is relevant in the communication system, a brief review of the interaction of radio frequency energy with objects is in order.
To communicate at higher data rates, through a wireless channel and simultaneously have the ability for that energy to communicate through physical barriers such as buildings, walls, foliage, soil or even through tunnels, the spectral energy should have a fair amount of “color” to minimize the risk of having a particular frequency, or band of frequencies, blocked. There are two general advantages to including low frequencies in the transmitted signal. A first advantage is that low frequencies are able to penetrate lossy medium. This is why the United States Navy uses very low frequency radio frequency transmissions to communicate through sea water to submarines. This penetration phenomena may be viewed as a “skin effect”, where attenuation is proportional in decibels to the frequency of the transmission. An example of the effectiveness with which lower frequencies penetrate structures as compared to high frequencies is shown in FIG. 1. In FIG. 1, the amount of attenuation in dB is shown to be related to frequency of the radio frequency energy for a variety of different materials. One way to view this is that the attenuation of lower frequencies minimizes the amount of reflection. Generally objects must be sizable (larger than a quarter of the wavelength) to reflect the wave. Accordingly, many smaller objects reflect higher frequency microwaves, but the same small objects do not interact with the lower frequency waves because they are too small relative to the wavelength of the transmitted signal.
Therefore, as recognized by the present inventors, there is a need to have a communication system that can include a spectral component where penetration occurs. Current spread-spectrum and narrowband systems cannot coexist with other narrow bandwidth users of the same spectrum due to mutual interference (DMA, overlaps, spectrums for user's, but strict power control must be adhered to. Too much interference is impinged on the other users, who themselves cause too much interference to the communication system. Typically, high-speed links operate on microwave carriers that are easily blocked by terrain and intervening objects. Such systems rely on all components (e.g. the antenna) having a reasonably flat frequency response over the bandwidth used, and therefore do not affect the waveform. They also assume that there are several to many cycles of the carrier between transitions (e.g. zero crossings) in the modulating waveform.
These conventional narrowband modulation schemes (narrowband including traditional direct sequence and frequency hopping spread spectrum system) are considered to be narrowband because at most only 10 or 20% of the carrier frequency is reflected in a spectrum of the modulated waveform. The bandwidth then is a narrow frequency range containing 90% of the energy spanning Fl (the lowest frequency) to Fh (the highest frequency). If the center frequency is Fc, and considered to be (Fl+Fh)/2 and referred to as “the carrier” frequency, then the bandwidth that may be employed in a UWB system can exceed 100%, a seemingly impossible number for conventional “narrowband systems.” It is the recognition of this fact by the present inventors that allows the present invention to operate simultaneously at low, penetrating frequencies, yet still be able to resolve multipath (reflected signals) and maintain high data rates.
It is the recognition by the present inventors of these phenomena that allows the present invention to simultaneously operate at low frequencies, yet resolve multipath, and maintain high data rates. This combination has substantial benefits because low frequencies both penetrate lossy media and minimize reflections off objects because they become smaller relative to the wavelength. In contrast, conventional systems typically have less than 10% bandwidth, and therefore have poor resolution at low frequencies.
Other UWB systems have been based on producing and receiving short one-to two cycle impulses at a relatively low duty cycle. Examples include deRosa (U.S. Pat. No. 2,671,896), Robbins (U.S. Pat. No. 3,662,316), Morey (U.S. Pat. No. 3,806,795), Ross and Mara (U.S. Pat. No. 5,337,054), and Fullerton and Kowie (U.S. Pat. No. 5,677,927). Impulses on the order of 1 ns are emitted at a 1 to 10 MHZ rate, giving rise to a 100:1 to 1000:1 duty cycle.
As presently recognized, this low duty cycle causes two problems. First, it is difficult, or nearly impossible, to generate significant average power efficiently due to high peaks. For example, because the peak voltages are higher than breakdown voltage of state-of-the-art components, low-voltage (1.8 V) CMOS in bipolar processes, standard low cost implementations are limited. Second, the high peaks disrupt “crystal detector” receivers, which are sensitive to time-domain space.
In contrast, the waveform used in the present invention is constructed from sequences of shape-modulated wavelets (i.e. short, spatially compact, impulsive, electromagnetic wavelets) with an energy envelope resembling a single smooth Gaussian pulse. Transmission of this high duty cycle waveform solves both problems. Low voltage parts can easily create the required waveforms, and the transmitted energy is spread in both time and frequency so that it looks like noise in all domains. Analysis procedures like JTFA (joint time-frequency analysis) often plot an image of the signal energy where time is along the x-axis, frequency is along the y-axis, and bright spots represent high energy at a particular time and frequency. Often several images are made, each with different trades between frequency resolution for time resolution. In low duty-cycle UWB systems, these images appear as vertical bars of somewhat random spacing. In other higher duty cycle UWB systems, like Fleming and Kushner (U.S. Pat. No. 5,748,891), these images have a structured appearance, for example, like a moray pattern. Using a bi-phase waveform according to the present invention, the transmitted signal appears smooth in these images.
Conventional systems also use pseudo-random time intervals between unchanging (essentially identical) pulses, for the purpose of spreading the spectrum and conveying information. Moreover, these systems use pulse position modulation (PPM) to convey information. As presently recognized, this way of communicating information, however, is sub-optimal for several reasons. PPM is sub-optimal in a multipath channel because the demodulator can mistake an illegitimate time shift done by a multipath reflection, for a legitimate time shift done by the modulator. By contrast, the present invention communicates information by changing the pulse shape. Therefore, all multipath is stationary relative a pulse conveying information. Thus, multipath is not confused with data modulation.
Another reason PPM is sub-optimal is that the error probability is high for a PPM detector given a signal with added noise. Analysis of conventional systems using coherent BPSK and PPM shows that for identical bandwidth channels and equal data-rate and bit-error-rate (BER), BPSK can tolerate approximately 6 dB greater noise. Even in its simplest form (a single wavelet coded by inverting or not inverting) the present invention captures the same 6 dB advantage. Part of the reason for this is that a single pulse is necessarily shorter than a window in which one can sense a pulse in two positions. In the present invention, the duration of a single pulse represents a time slot to convey information. In PPM systems, approximately 1.4 to 2 pulse widths represent a time slot to convey information. Another reason is that the difference in voltage, between a detected “one” and a detected “zero”, is smaller than that of the BPSK signal. Consequently, it takes more signal power to get over the noise.
Whitening the transmitted spectrum from a UWB system is imperative if it is to not interfere with other users of the same spectrum. Yet another difference between conventional systems and the present invention is the interference generated by the transmitter. While the present invention allows the pulse position to be randomized for the purposes of spectrum control (i.e. to make the output power spectrum smooth), it does not require it, nor does it use it in the preferred embodiment. Instead, the spectrum is smoothed by generating random-looking sequences of shape-modulated pulses such that the waveform as a whole appears random. A transmitter according to the present invention transmits a “symbol” from one of a family of sequences of pulse shapes, where each “symbol” may communicate more that one bit of information, and the series of information bearing “symbols”, creates an overall waveform that is “whiter” than conventional systems. As a result, transmissions from transmitters according to the present invention will cause less interference than conventional systems, even if both systems were broadcasting identical average power over essentially identical bandwidths. When added to the aforementioned 6 dB advantage over PPM, the present invention can offer equal communication rates and bit-error-rates, BER, at far less interference levels.
The flip side of whitening the transmitted spectrum for a UWB system is the impact it has on reception. To spectrally whiten the transmitted spectrum, conventional UWB systems jitter the time spacing between pulses. This jittering has severe consequences that are avoided in the present invention. To explain by example, a sine wave that is sampled at random times appears to be “noise.” Similarly, any spectral peaks (i.e. tones, or near sine waves such as all conventional narrowband emissions) entering conventional UWB receivers appears as noise in the data samples. To communicate, the desired signal must be strong enough to overcome this “noise.” The preferred embodiment of the present invention does not jitter the pulse position. Instead, pulses are identically spaced according to a precise clock. Consequently, tones entering the receiver are captured such that they appear as a pattern in the data samples. As discussed herein, this pattern can be recognized, estimated, and subtracted such that interference caused by the tones is largely removed. Even in the case where the tone entering the receiver is above a Nyquist cutoff (i.e. at a frequency higher than half that of the data samples), frequency folding occurs such that a pattern still occurs. This feature allows the present invention to operate in high noise environments at ranges and data rates far beyond that of conventional UWB systems because the receiver operates with equivalently, less noise.
As presently recognized, it is desirable to have a high data rate in a channel with a high degree of multipath. Conventional systems are limited by intersymbol interference caused by multipath. Buildings, for example, give rise to particularly bad multipath (i.e., interfering echo signals) occurring out to about 500 ns after the direct path signal. Therefore, sending pulses spaced closer that 500 ns to obtain higher data rate, only serves to introduce greater intersymbol interference. The present invention solves this difficult problem by transmitting symbols that communicate more that one bit of information. Since each symbol is itself, spectrally white (meaning its autocorrelation is a spike with low sidelobes), multipath continues to be resolved over the duration of the symbol. Therefore, high data rates can be obtained without intersymbol interference even in the presence of high multipath.
A feature of present invention is that by transmitting one of a family of pulse shapes, each pulse may communicate more that one bit of information, yet not lose any of the aforementioned benefits. To communicate more than one bit per pulse in a PPM system, more time-slots could be used. The multipath degradation described earlier, however, would be severely aggravated, plus the other problems accentuated too. Conventional UWB systems lack control over the shape of their waveform, and are unable to transmit multiple bits per pulse.
Prior art UWB communication systems require high precision clocks to reduce the time it takes to acquire synchronization. Even with precision clocks, acquisition times are often measured in tens of seconds. This reduces the realized data rate and makes the devices difficult to use. A feature of present invention is that synchronization can be obtained quickly, often measured in ms.